What are things I can do that will almost guarantee acceptance into medical school?
Unfortunately there is nothing you can do that will "almost guarantee" your admission. The best you can do is improve your odds.
In general, you should have all of the following:
1) a high GPA and sGPA (3.6 or above) while taking at least 12 credits/semester
2) a strong evenly balanced MCAT score with no subsection score below 8
3) basic lab or clinical research experience
4) community service (the more the better, esp. a long-term deep commitment to a single issue)
5) clinical volunteering (at a hospital, free-standing clinic, nursing home, group home to demonstrate your committment to medicine)
6) physician shadowing (to better understand what it is a physician does and how they interact with patients and other members of the medical staff)
7) leadership positions in your ECs
8) excellent oral and written communication skills
9) excellent interpersonal skills
10) a non-judgmental attitude towards others
11) maturity together with a cultivated sense of personal humility (nothing turns interviewers off faster than an applicant that comes off as entitled, arrogant or a$$holish, or one that is "gaming the system")
12) hobbies, interests and passions that lie outside of medicine (makes you a more interesting person/applicant)
However, the truth is that you can have all these things and still not get accepted into medical school. Every year thousands of viable applicants are denied admission simply because there aren't enough seats available.
Medicine is a very long haul. Good luck!